The best way to paint is actually take the tyres and valve pin off the wheel. The scrape out all the existing paint, then the below process needs to be done

1. Primer Coat
2. Heat it in the oven for about 70 Degree C
3. After the wheel cools down, apply the actual paint
4. The heating process starts again
5. Second coat, if required and then the heating process
6. After the wheel cools down, the clear coat needs to applied
7. Final round of heating and cooling process.
8. The cleaning of alloys and putting it back with the tyres on to the car.

All the wheels are supposed to be painted at a time to get a near perfect color match.